The United States is sending a senior envoy to North Korea to seek freedom for an ailing Korean-American Christian missionary who has been sentenced to 15 years at hard labor.

The State Department says Special Envoy for North Korean Human Rights Issues Robert King will ask that Kenneth Bae be freed on humanitarian grounds when he visits Pyongyang on Friday.

A White House spokesman says the United States is deeply concerned about Bae, who is reported to have been moved from a labor camp to a hospital after losing more than 23 kilograms. The 44-year-old suffers several health problems, including with his liver and kidneys.

Bae was arrested after entering the country in November as a tour operator. He was later convicted of trying to topple the government.

North Korea has not publicly responded to the news of King's trip. But the State Department says his visit is being made at the invitation of the Pyongyang government.

Council on Foreign Relations senior fellow Scott Snyder believes an agreement on Mr. Bae's release has already been worked out.

"Still hard to say it's already been agreed to or not, but I think that it is likely that Ambassador King would not go unless he was able to bring Kenneth Bae back.

North Korea has detained at least six Americans since 2009. All have been allowed to return home before serving their full sentences. Most were released following visits by prominent Americans, including former presidents Jimmy Carter and Bill Clinton.

Euna Lee, a Korean-American journalist who was freed in 2009 after a special visit to Pyongyang by Mr. Clinton, told VOA's Korean service that she is grateful for those working to bring Mr. Bae home.

"I am on the verge of crying. I can barely speak. I am so thankful to those who worked on his and his family's behalf," she said.

In a video earlier this month posted by a pro-North Korea newspaper in Japan, Bae said his health was deteriorating and called on the U.S. to send a high-ranking official to negotiate his release.

Many analysts have said Pyongyang is using Bae's case as a bargaining chip to try to extract concessions from the U.S. over its nuclear and missile programs. North Korea denies this.

Bae's arrest came at a time of heightened U.S.-North Korea tension. Just weeks after his arrest, Pyongyang carried out a long-range rocket launch and subsequent nuclear test, both of which were strongly condemned by the United Nations.

At the height of the crisis, North Korea was threatening to launch nuclear attacks on the United States and South Korea. Tensions have since died down, with Pyongyang taking several steps to improve relations with Seoul.

The U.S. statements on Tuesday did not mention whether any effort would be made during the trip to advance multi-nation nuclear disarmament talks, which have been stalled since 2009.

King played a part in negotiating a 2012 deal under which North Korean leader Kim Jong Un agreed to freeze his impoverished country's nuclear program in exchange for food aid. Washington later suspended the agreement after North Korea launched a rocket.